Splitter control looks by default like on the picture below. Its purpose to allow changing the size of aligned controls by mouse down and drag. It only wotks with controls which alignment differs from alNone. For example, you may place left aligned group box on a form, then place a left aligned splitter, and then place client aligned panel. It will allow user to change areas occipied by group box and panel on a form simply by dragging splitter.

Splitter control(widget)